tipc: Consolidate subscriber & subscriber port references

This patch modifies TIPC's network topology service so that it
only requires a single reference table entry per subscriber
connection, rather than two.  This is achieved by letting the
reference to the server port communicating with the subscriber
act as the reference to the subscriber object itself.  (Since
the subscriber cannot exist without its port, and vice versa,
this dual role for the reference is perfectly natural.)  This
consolidation reduces the size of the reference table by 50%
in the default configuration.

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>